1. Artificial Oxygen Carriers
Artificial oxygen carriers/transporters are an extremely heterogeneous group of substances. Their name-giving characteristics are their ability to bind oxygen in the form of molecular dioxygen (O2) reversibly or to dissolve it—thus in principle they have a property in common with the natural oxygen carrier/transporter in the blood, hemoglobin (red blood pigment) that occurs in the erythrocytes (red blood cells)—and their potential usefulness as pharmaceuticals to be administered intravascularly (usually intravenously), or in other biomedical applications.
(A comprehensive review (state of the art) in: RIESS J. G.: “Oxygen Carriers (“Blood Substitutes)—Raison d'Etre, Chemistry, and some Physiology,” Chemical Reviews 101 (2001): 2797-2919; a review of many hemoglobin derivatives in: VANDEGRIFF K. D.: “Haemoglobin-based Oxygen Carriers”: Expert Opinions on Investigational Drugs 9 (2000): 1967-1984).
The known oxygen carriers differ both with respect to their nature and with respect to the resultant physicochemical properties and their usability.
Thus, perfluorocarbons are immiscible with and insoluble in aqueous solutions, such as blood plasma for example. However, they can be emulsified therein in the form of finely dispersed droplets (stabilized with emulsifiers). Liposomes filled with natural or artificial oxygen carriers are likewise emulsified or suspended. These are vesicles (artificial cells or artificial erythrocytes) surrounded by a phospholipid double layer membrane.
Hemoglobins, their derivatives obtainable by chemical modification, and isolated and necessarily chemically modified heme groups can be dissolved freely in the aqueous phase (in plasma, for example).
The molecular structure of artificial oxygen carriers determines their method of administration, especially whether they can be substituted as a replacement for missing blood, or whether they can be added to existing blood as an additive. Products described up to now are intended to be oxygen-transporting plasma substitutes, or a plasma replacement fluid to fill up the vascular system partially drained by acute hemorrhage or by blood withdrawal, which in contrast to the known (non-oxygen-transporting) plasma substitutes also restore another essential function of the blood, namely oxygen transport.
Perfluorocarbons and liposomes do not dissolve in aqueous blood plasma; as a distinctly separate emulsified or suspended phase of their own, they have and they occupy a certain volume, and therefore they seem suitable in principle for the mentioned purpose as oxygen-transporting plasma substitutes, but on the other hand not as additives to the blood since they necessarily increase its volume.
To be suitable as a replacement for missing blood, oxygen-transporting plasma substitutes of hemoglobins or of their derivatives obtained by chemical modifications freely dissolved in an aqueous phase have to be both isotonic (tonicity is a relative measure of osmotic pressure) and isoncotic (=iso-oncotic; oncoticity is a measure of the oncotic (=colloidal-osmotic) pressure) with the blood plasma. To produce isotonicity, such artificial oxygen carriers are usually dissolved in an electrolyte solution that resembles blood plasma electrolytes.
Hemoglobin derivatives developed (and published) up to now as artificial oxygen carriers themselves involve iso-oncoticity in pharmaceutical preparations. Their molecular design conforms to the clinical requirement for iso-oncoticity, which is accomplished by a sufficient number of oncotically active drug molecules.
For this reason, such freely dissolved hemoglobin derivatives are also very particularly proposed for use in case of (severe) blood loss. They are only very conditionally usable (namely extremely limited in amount/dose) for medical indications without blood loss, since because of their mentioned properties, they necessarily increase blood volume by the volume of their injected or infused pharmaceutical preparation.
2. Hemoglobin Hyperpolymers
If artificial oxygen carriers are to be used as additives to treat oxygen deficiency, they should have a sufficiently low colloidal-osmotic pressure (cf. Barnikol W. K. R. et al. (1996): “Hyperpolymeric hemoglobins as artificial oxygen carriers—an innovative approach to medical development,” Therapiewoche 46: 811-815). They are planned as artificial oxygen carriers to increase the oxygen transport capacity of existing blood when no blood loss is to be replaced. For hemoglobin hyperpolymers after injection or infusion not permanently to increase the volume of circulating blood (but instead for the water and the salts of their preparation to be extensively excreted again through the kidneys), the oncotically active number of drug molecules has to be reduced as much as possible. To this end, the hemoglobins are crosslinked and polymerized chemically (by means of polyfunctional or bifunctional crosslinking agents). Giant artificial oxygen-binding molecules are formed in this way. From the chemical viewpoint, molecularly crosslinked hemoglobins are multimers of the monomer. However, this says nothing about what multimers—and this involves a broad distribution of molecular weights with oligomers and higher polymers—have what effects on the properties of the overall product.
3. Pulmonary Edema
Edema is an abnormal fluid accumulation in the intercellular space (interstitium). Pulmonary edemas are a frequent clinical syndrome. They lead to a life-threatening impairment of health that leads to death in severe cases. Distinction is made principally between cardiac (obstructive) edema caused by insufficiency of the left ventricle and pulmonary edema of toxic genesis from elevated capillary permeability with pulmonary inflammation, inhalation of injurious gases, for example, also from high oxygen concentrations, uremia, or hypersensitivity reactions, etc.
Therapy is always symptomatic with regard to the life-threatening impairment of pulmonary function (intensive medical care, corticoids to suppress inflammatory processes, oxygen-enriched respiratory air, and pressurized respiration, etc.), and if possible causal with regard to the causes (exposure prophylaxis, therapy for cardiac insufficiency, or for the underlying kidney disease, etc.).
(For the state of the art, for example, see: Böcker, W., Denk, H., Heitz Ph. U (Ed.): Pathology, Urban & Schwarzenberg, Munich and elsewhere 1997; Gerock W., Huber C H, Meinertz T., Zeidler, H. (Ed.): Gross•Schölmerich•Gerock—Die Innere Medizin, 10th completely new revision and expanded edition, Schattauer, Stuttgart and New York 2000; Weikrauch, T. R. (Ed.): Wolff—Weikrauch—Internistische Therapie 2000/2001, 13th revised edition, Urban & Fischer, Munich, and Jena 2000; Arch. Cardio. Mex., Vol. 72, pages 280-285).